Opto-electronic scale reading apparatus with differing optical path lengths

ABSTRACT

A readhead (18) measures movement in the direction of arrow (20) with respect to a scale (10). The readhead (18) comprises an index grating (12), which interacts with light from the scale (10) to produce fringes in the vicinity of an analyzer grating (14). The resulting light modulations are detected by photodetectors (16). In order to reduce the sensitivity of the apparatus to changes in the ride height (h) of the readhead above the scale, at least two different optical path lengths are provided for light travelling from the scale (10) to the index grating (12), and/or from the index grating (12) to the analyzer (14). Various ways to achieve this are disclosed, e.g. a plate (22) of a refractive medium placed in one optical path, or tilting one of the gratings.

FIELD OF THE INVENTION

This invention relates to opto-electronic scale reading apparatus. Suchapparatus nay be used with a scale to determine the magnitude anddirection of movement of one member relative to another. Such anapparatus is typically used to measure the relative movement of twomembers on coordinate positioning machines such as machine tools orcoordinate measuring machines.

DESCRIPTION OF PRIOR ART

A known type of such device is described in U.S. Pat. No. 4,959,542.Here, an elongate scale which can be fitted to one of the above memberscomprises a series of marks, in the form of parallel lines, spaced apartin the longitudinal direction of the scale. A readhead for attachment tothe other of the members includes an index grating and an analysergrating. Light reflected from (or transmitted through) the scaleinteracts with the index grating, and causes the generation of a fringepattern in the plane of the analyser grating. When the scale andreadhead are moved relative to each other, this fringe pattern moves ina corresponding fashion in the plane of the analyser grating, andphotodetectors situated behind the analyser grating receive a modulatedlight signal, from which the distance and direction moved can bedetermined.

Alternatively, our European Patent Application No. EP 543513 disclosesthat the analyser grating and the detectors may be combined, andprovided on a single semiconductor substrate. This combined grating anddetector may be referred to as an electrograting.

The arrangement of U.S. Pat. No. 4,959,542 provides a spatial filteringeffect which ensures that the readhead is very tolerant of defects inthe scale or dirt which may accumulate on the scale. However, to ensurethat fringes with good contrast are produced in the plane of theanalyser grating, it is necessary to hold the spacings between thescale, the index grating and the analyser grating substantially inaccordance with formulae which are given in the specification of U.S.Pat No. 4,959,542. These formulae relate those spacings to the pitchesof the scale, the index grating and the analyser grating.

As a result, it is found in practice that the readhead is fairlysensitive to the offset or "ride height" between the scale and thereadhead. This is illustrated in FIG. 1 of the accompanying drawings,which shows a sensitivity curve S illustrating how the amplitude A ofthe signal output from the readhead varies with the offset or rideheight h of the readhead above the scale. It will be seen that a strong,usable signal is obtained only within a relatively narrow tolerance bandT. When the scale and readhead is installed on a machine, therefore, itis necessary to ensure that the readhead will remain within thistolerance band T throughout the length of its travel along the scale.

British Patent No. 1,474,049 shows scales and readheads of a type whichis different from those discussed above. In one embodiment, light passesfrom a first grating to a second grating, which reflects it back to thefirst grating. The second grating is tilted with respect to the firstgrating so that the return optical path from the second grating to thefirst is greater than the outward optical path from the first grating tothe second. U.S. Pat. No. 4,636,076 shows a development, in which thereturn optical path differs from the outward optical path by virtue ofthe light passing through different thicknesses of a material having arefractive index different from the ambient medium. However, these twopatents do not address the problems with which the present invention isconcerned.

SUMMARY OF THE INVENTION

The present invention provides a scale and readhead apparatus,comprising:

an elongate scale defined by a series of marks spaced apart in thelongitudinal direction;

a readhead comprising an index grating and an analyser, light travellingfrom the scale to the index grating and analyser;

characterised in that at least two different optical path lengths areprovided for light travelling from the scale to the index grating,and/or from the index grating to the analyser.

With such an arrangement, light from the scale may interact with theindex grating and cause a fringe pattern in the vicinity of theanalyser. However, by virtue of the different optical path lengths(which vary across the field of view of the analyser), for a given rideheight of the readhead above the scale, only the fringe pattern causedby light travelling along one of the optical paths coincides with theanalyser to give a good output signal. At another given ride height, thefringe pattern caused by light travelling along another one of theoptical paths coincides with the grating to give a good output signal.Thus, good output signals are provided at more than one ride height.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described by way ofexample, and with reference to the accompanying drawings, wherein:

FIG. 1 is a graph showing the sensitivity of a prior art readhead to theride height above a scale;

FIG. 2 is a graph similar to FIG. 1, but showing the ride heightsensitivity of an embodiment of the present invention;

FIGS. 3-18 are schematic longitudinal sectional views through scales andreadheads according to various embodiments of the invention;

FIG. 19 is a plan view of the scale and readhead according to FIG. 3;

FIG. 20 is a plan view corresponding to FIG. 19 but showing amodification;

FIG. 21 is a schematic sectional view and block diagram of anotherembodiment;

FIG. 22 is a schematic longitudinal sectional view through a furtherembodiment; and

FIG. 23 is a transverse section on the line A--A in FIG. 22.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 shows a scale 10, index grating 12, analyser grating 14 and oneor more photodetectors 16. The scale comprises a series of marks, in theform of parallel lines, spaced apart in the longitudinal direction ofthe scale. The index and analyser gratings 12,14 and the photodetectors16 are provided as a unit in a readhead 18 (together with a lens and alight source for illuminating the scale, neither of which is shown). Thearrangement is generally as disclosed in U.S. Pat. No. 4,959,542.

In that patent, it is described that the spacings between the scale,index grating and analyser are related to the pitches of the marks orlines of these optical elements by certain formulae. This causes thelight reflected from the marks on the scale to interact with the indexgrating to form fringes in the plane of the analyser grating. Thesefringes move in accordance with relative longitudinal movement betweenthe scale and the readhead, and this can be detected in several ways.

For example, the analyser may be slightly skewed relative to the indexgrating, within its own plane (which still remains parallel to the indexgrating). The fringes formed in the plane of the analyser then in turninteract with the analyser to generate moire fringes. There may then bethree or four photodetectors 16, arranged transversely with respect tothe scale in order to detect the moire fringes. Their outputs may becombined in such a way as to give two signals in quadrature.

Alternatively, the analyser 14 and photodetectors 16 may be combined andprovided on a single semiconductor substrate in the form of anelectrograting, as described in European Patent Application No. EP543513. Such an electrograting comprises a linear array of narrow,finely spaced photodetectors which take the place of a conventionalgrating. Interdigitated sets of the photodetectors are each connected incommon to provide the required output signals. As stated in EP 543513,the electrograting may also include integral light sources, or aseparate light source may be provided. A lens in the readhead is notnecessary.

The readhead 18 rides at a height h above the scale 10 as it travelslongitudinally along the scale in the direction indicated by arrows 20.If the readhead 18 consisted only of the gratings 12,14 and detectors 16as described thus far, then it would be fairly intolerant of changes inthis ride height h as shown in FIG. 1 and discussed above.

To ameliorate this problem, FIG. 3 shows a plate 22 of a transparentrefractive medium inserted between the index grating 12 and the analysergrating 14. However, this plate 22 covers only half of the working areaor aperture of the analyser grating 14, as seen also in the plan view ofFIG. 19. Because it has a different refractive index from thesurrounding air, the plate 22 means that the optical path length betweenthe scale 10 and the analyser 14 on the right hand side of the analyser(as seen in FIGS. 3 and 19) differs from that on the left hand side. Inconsequence, the plane 15A of the fringes formed by the interactionbetween the scale and the index grating on the right hand side of thesefigures differs from the plane 15B of the corresponding fringes on theleft hand side.

Another way to view this is to consider the ratio u/v, where u is theoptical path length from the scale 10 to the index grating 12, and v isthe optical path length from the index grating 12 to the analyser 14.Because the plate 22 on the right hand side of FIGS. 3 and 19 has alarger refractive index than the air in the corresponding position onthe left hand side, the optical length v is larger on the right handside. Thus, the ratio u/v is different on the right hand side, comparedwith the left hand side.

This affects the ride height sensitivity in the manner shown in FIG. 2.In place of the ride height sensitivity curve S shown in FIG. 1, we nowhave two such curves, S1 and S2. One of these curves corresponds to theoptical path length on the right hand side of FIG. 3, i.e. when the rideheight h is such that the fringes in the plane 15A coincide with theplane of the analyser 14. The other curve corresponds to the opticalpath length on the left hand side, i.e. when the ride height h is suchthat the fringes in the plane 15B coincide with the plane of theanalyser 14. Each of these curves has a smaller height than the curve Sin FIG. 1, but the overall sensitivity of the readhead is now as shownby the broken line S3.

It can be seen that this overall sensitivity curve S3 is broader thanthat in FIG. 1, even though it is not as high. The result, therefore, isthat the readhead gives useable signals over a tolerance band T1 whichis wider than the tolerance band T. In other words, the readhead is lesssensitive to changes in the ride height h compared with the prior artarrangement. It will be noted that the plane in which the fringes areformed no longer needs to coincide exactly with the plane of theanalyser, but rather the planes 15A, 15B are merely in the vicinity ofthe plane of the analyser 14.

The photodetector or photodetectors 16 should have a width (in thelongitudinal direction) sufficient to straddle both of the opticalpaths, so as to detect the light signals from the fringes in bothoptical paths in common and produce an output signal which combines thelight signals. However, if the fringes in the planes 15A, 15B aredetected by producing moire fringes, then the detectors should not be sowide as to be unable to distinguish these moire fringes. Alternatively,there may be a detector or a set of detectors for each of the opticalpaths, their outputs being combined so as to detect the light signals inthe two optical paths in common. If an electrograting according to EP543513 is used, then the array of interconnected photodetectors willautomatically straddle the two optical paths and detect the lightsignals in these paths in common.

FIG. 4 shows an alternative arrangement, in which the refractive plate22 is provided with a step in a region 24. Effectively, there are nowthree different optical path lengths between the scale 10 and theanalyser grating 14, namely on the left hand side of the Figure (wherethere is no plate 22); in the central, stepped region 24 (where theplate 22 is relatively thin); and on the right hand side of the Figure(where the plate 22 is thicker). This has the effect of adding anotherpeak to the peaks S1 and S2 in FIG. 2. It will be appreciated that ifone merely made the plate 22 of FIG. 3 thicker, without a stepped region24, then the peaks S1 and S2 in FIG. 2 would be further apart,broadening the tolerance band T1, but creating a significant dip in theoverall sensitivity S3 between the two peaks S1 and S2. The effect ofthe stepped region 24 is to provide a third peak which fills this dip.

FIG. 5 shows another arrangement. Here, the plate 22 extends across theentire width of the analyser grating 14, but has a castellated profile26. FIG. 6 shows a similar arrangement, but with an undulating or ribbedprofile 28. In both FIGS. 5 and 6, the castellations or ribs should havea pitch much greater than the scale and the gratings 12,14, to preventdiffraction and interference effects.

FIG. 7 shows an alternative, in which a refractive medium 30 having avarying thickness is deposited over the surface of the analyser 14. In asimilar manner, such a coating 30 may be provided over a conventionalanalyser grating. These configurations may be more advantageous thanthat of FIG. 6, since the normal operation of the readhead is lesslikely to be impaired by refraction at the undulating surface.

FIG. 8 shows that a refractive medium of varying thickness may insteadbe deposited on the surface of the scale 10. Once again, in any of thesecases, the undulations in the thickness of the refractive medium 30,32should be on a much larger scale than the pitch of the scale and thegratings, to avoid diffraction and interference effects.

FIG. 9 shows an alternative in which the substrate of the index grating12 is formed with steps 34, giving the same effect as the insertion ofthe plate 22 in FIGS. 3 and 4. There may be any suitable number of suchsteps 34.

FIG. 10 shows how the same effect can be achieved by having an indexgrating substrate which is wedge-shaped, i.e. having a continuallyvarying thickness instead of a stepped thickness. This results in afringe plane 15 which is tilted with respect to the analyser 14. Fordifferent ride heights, a different point on the analyser 14 willcoincide with the plane 15 of the fringes.

FIG. 11 shows a similar arrangement, but using a wedge-shaped refractiveplate 22 between the index grating 12 and the analyser grating 14. FIG.12 shows the same arrangement, but the plate 22 is now in the form of abiprism or double wedge.

Of course, it is possible to insert the plate 22 of FIG. 3 between thescale 10 and the index grating 12 instead of between the index grating12 and the analyser 14. This is illustrated in FIG. 13. Any of theplates 22 shown in FIGS. 4, 5, 6, 11 or 12 may similarly be insertedbetween the scale 10 and the index grating 12. The effect is to vary theoptical length u (FIG. 3) instead of the optical length v.

FIG. 14 shows another embodiment. Instead of varying the optical pathlength in different regions of the analyser grating 14, the analyser 14is instead vibrated or oscillated towards and away from the indexgrating 12, as indicated by the arrows 36. Thus, we now have differentoptical paths at different times, rather than in different regions. Thevibration may be achieved by, for example, a small motor or apiezoelectric actuator. The frequency of the vibration should besubstantially higher than the frequency of the light modulationsproduced by the movement of the scale relative to the readhead, so thatthe signals over one or more cycles of the vibration can be integrated(e.g. by a suitable filter circuit). This ensures that the light signalsfrom the different optical path lengths (i.e. at different times) aredetected in common, as previously. The effect of this vibration is thatthe single sensitivity peak S (and the corresponding tolerance band T)in FIG. 1 vibrates in the horizontal direction of that Figure, giving aneffective broadening of the ride heights h to which the readhead isresponsive.

FIGS. 15-17 illustrate further embodiments. Here, an optical path lengthratio u/v which varies gradually from the left hand side to the righthand side of the analyser is achieved by deliberately tilting one of theelements so that its plane is no longer parallel to the others.

In FIG. 15 the analyser 14 is tilted. At one ride height of the readheadabove the scale, fringes will formed in the plane 15C; while at anotherride height they will be formed in the plane 15D. These and similarplanes each coincide with a different point on the tilted analyser, sothat good signals can be produced from the region around the point ofcoincidence for various different ride heights.

In FIG. 16 the index grating 12 is tilted. This results in the plane 15of the fringes also being tilted. Again, for different ride heights, adifferent point on the analyser 14 will coincide with the plane 15 ofthe fringes. A similar effect is achieved in FIG. 17, in which the scale10 is tilted. The tilting of the scale 10 in FIG. 17 may in practice bemost conveniently achieved by tilting the entire readhead relative tothe scale.

In FIGS. 4-17, the photodetector or photodetectors have been omitted forsimplicity. However, it will be understood that, as in FIG. 3, they maybe provided behind the analyser grating 14, or alternatively anelectrograting may be used as the analyser 14. The above comments aboutdetecting the signals from the various optical paths in common should beobserved.

A further embodiment is shown in FIG. 18. Here, the index grating 12 isprovided on the underside of a beam splitter cube 38. Two separateanalyser gratings (and the corresponding detectors) or two separateelectrogratings 14A, 14B are provided, respectively receiving the splitbeams produced by the beam splitter 38. The detectors of the analysers14A, 14B are connected to the signal processing circuit of the readheadin parallel, to detect the light signals in common. To produce therequired differing optical path lengths, one of the analysers 14B may belocated further from the centre of the cube than the other analyser 14A.Desirably, the optical path difference can be accentuated by inserting alayer 40 of a refractive medium such as glass before the analyser 14B.This layer 40 may suitably be formed on the surface of the cube 38.

The embodiments described above have been illustrated by sectional viewstaken in the longitudinal direction of the scale 10. Thus, the opticalpath length varies for different positions in this longitudinaldirection. For example, FIG. 19 shows how the plate 22 of FIG. 3 coversthe analyser grating 14 for only part of its longitudinal extent.Nevertheless, it is equally possible for the optical path length to varywith the transverse position, instead of the longitudinal position.

For example, FIG. 20 is a plan view of a modification of the arrangementof FIGS. 3 and 19. The refractive plate 22 in FIG. 20 is insertedbetween the scale 10 and the analyser grating 14 in such a manner as tocover the grating 14 for only part of its transverse extent, rather thanfor part of its longitudinal extent. The arrangements of FIGS. 4-13 and15-18 may be modified similarly. For example, the steps, castellationsand ribs of FIGS. 4-9, the wedge profiles of FIGS. 10-12, and the beamsplitter of FIG. 18, may each be turned through 90°.

In the case of FIGS. 15-17, it will be appreciated that the requiredtilting was shown as the deliberate introduction of a pitch anglebetween the tilted element and the other elements, giving the variationin optical path length in the longitudinal direction. It will beunderstood that the optical path length may be made to vary in thetransverse direction by introducing a deliberate roll angle to theappropriate element instead of a deliberate pitch angle.

One example of this is illustrated in FIGS. 22-23. Here, as seen in FIG.23, the index grating 12 is tilted with a suitable roll angle, producingfringes in a tilted plane 15. The analyser 14 is an electrogratingaccording to EP 543513. A separate light source 50 is provided, in theform of an infra-red light emitting diode. This could illuminate thescale 10 directly, as in the other embodiments, but for convenience ofdesign and manufacture it is arranged to illuminate it via the indexgrating 12. By locating the light source close to the index grating itis possible to prevent fringes from forming on the scale itself. It willbe seen from FIG. 22 that the light source 50 provides the illuminationobliquely from one end. This is ideal when the scale has a profile asshown in U.S. Pat. No. 4,974,962, where the scale marks have facetswhich reflect the light vertically towards the index grating 12 andanalyser 14.

FIG. 21 shows yet another embodiment. Like FIG. 14, the optical pathlength is here varied by moving the analyser grating 14 towards and awayfrom the scale 10. Thus, there are different optical paths at differenttimes, instead of in different regions. Rather than the continuousvibration of FIG. 14, however, the optical path length is varied byaservo loop. The outputs from the detectors 16 are processed in a signalprocessing circuit 42, which (amongst other outputs) produces an output44 which varies with the amplitude or intensity of the outputs from thedetectors 16. This output may be produced by rectifying and smoothingthe relevant output signals, and combining them together. Alternatively,it may be produced by rectifying and smoothing the outputs of just oneof the detectors 16. The signal 44 is used to control a servo driver andmotor 46, which adjusts the position of the analyser grating 14 in sucha manner as to tend to keep the output signals from the detectors 16 atmaximum amplitude or intensity. The effect will be that, if for examplethe ride height h between the readhead and the scale 10 decreases thenthe analyser grating 14 will move closer to the index grating 12 tocompensate. This action will tend to keep the spacings between thescale, the index and the analyser gratings in accordance with thedesirable relationships explained in U.S. Pat. No. 4,959,542.

I claim:
 1. Opto-electronic scale and readhead apparatus, comprising:anelongate scale defined by a series of marks spaced apart in thelongitudinal direction; a readhead for movement in the longitudinaldirection of the scale, spaced therefrom by a ride height, the readheadcomprising an index grating and an analyser, light travelling from thescale to interact with the index grating, thereby producing fringes inthe vicinity of the analyser for analysis by the analyzer; characterisedin that at least two optical paths are provided, having differentoptical path lengths from the scale to the index grating, and/or fromthe index grating to the analyser; and for a first given ride height ofthe readhead above the scale, fringes caused by light travelling along afirst one of the optical paths coincide with the analyser; and fringestravelling along a second one of the optical paths do not coincide withthe analyser, while for a second given ride height of the readhead abovethe scale, fringes caused by light travelling along the second opticalpath coincide with the analyser, and fringe traveling along the firstoptical path do not coincide with the analyser.
 2. Apparatus accordingto claim 1, having at least one photodetector arranged for detectinglight from said different optical paths in common.
 3. Apparatusaccording to claim 1, wherein the analyser is in the form of asemiconductor device comprising a plurality of photodetectors. 4.Apparatus according to claim 1, wherein an element of a transparentrefractive material is located in at least one of said optical paths, toprovide different optical path lengths in said optical paths. 5.Apparatus according to claim 4, wherein said refractive element has adifferent thickness in different ones of said optical paths. 6.Apparatus according to claim 1 wherein one of the scale, the indexgrating and the analyser is tilted so as to be non-parallel to theothers, to provide different optical path lengths in said optical paths.7. Apparatus according to claim 6, wherein the index grating is tiltedso as to be non-parallel to the scale and the analyser, to provide saiddifferent optical path lengths.
 8. Apparatus according to claim 1,wherein:the ratio u/v in one of said optical paths differs from theratio u/v in another of said optical paths, where u is the optical pathlength from the scale to the index grating, and v is the optical pathlength from the index grating to the analyzer.